Spark gaps



Oct. 10, 1967 J. c. MARTIN SPARK GAPS 3 Sheets-Sheet 1 Filed Dec. 50, 1964 Oct. 10, 1967 I -.1. c. MARTIN 3,346,762

' SPARK GAPS Filed Dec. 50, 1964 5 Sheets-Sheet 2 FIG.3.' $2 5 $5? United States Patent 3,346,762 SPARK GAPS John Christopher Martin, Tadley, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Dec. 30, 1964, Ser. No. 422,127 Claims priority, application (great Britain, Jan. 3, 1964, 430 4 15 Claims. (Cl. 313-306) ABSTRACT OF THE DISCIJOSURE A spark-gap is provided with a trigger assembly comprising a trigger electrode having a sharp edge exposed to the gap, the sharp edge being separated by a thin insulating layer from a field-forming electrode. In operation the triggering voltage is applied between the trigger electrode and the field-forming electrode, the small separation producing an intense electric field at the sharp edge and thereby causing the production of corona at the sharp edge to initiate breakdown of the main gap, without breakdown occurring between the trigger and field-forming electrodes.

This invention relates to electrical spark-gaps of the type in which the discharge passes through a gaseous medium such as air.

Up to now the timing precision of switches of the gas gap type has been diflicult to control. It ha been the practice to use a large trigger pulse of high energy in order to achieve some precision. In some such switches the trigger pulse is applied to a trigger electrode to cause electrical breakdown between the trigger electrode and one of the switch electrodes as a consequence of corona initiation. This breakdown throws the full voltage on to the gap between the trigger electrode and the other switch electrode and the main gas discharge is initiated.

The present invention provides a spark-gap in which corona is initiated more readily, giving improved timing precision of breakdown whilst permitting the use of smaller trigger pulses and allowing a greater safety margin between the Working voltage and the breakdown voltage.

According to the present invention a spark-gap includes a trigger assembly arranged in spaced relationship with a main electrode of the gap, said assembly comprising a trigger electrode having -a sharp edge, a thin layer of solid dielectric material in contact with the sharp edge, and a field-forming electrode separated from said sharp edge by said thin layer.

Said assembly may comprise a member presenting a surface to said main electrode and adapted to be electrically conducting over at least a portion of said surface, said portion constituting said field-forming electrode, and a sharp-edged conductor overlying said portion but insulated therefrom by said dielectric layer to constitute said trigger electrode. Said member may be made of insulating material, said portion being a thin conducting sheet applied to said member. Said surface may be convex, and the sharp-edged conductor may bethin wire or preferably a thin conducting band on said dielectric layer.

One preferred form of the present spark-gap comprises two main electrodes presenting substantially spherical surfaces to said trigger assembly, the sharp-edge con ductor being a thin conducting band located substantially in and normal to a common diametral plane of the spherical surfaces. In a modified form the thin conducting sheet below a non-peripheral region of said band is recessed into said member to reduce the capacitance between band and sheet. A further thin conducting sheet may be insulatedly located between said thin conducting sheet and said member to form a capacitor therewith.

Another form of the invention comprises a pair of main spherical electrodes presenting substantially spherical surfaces to said trigger assembly wherein a laminar conducting member constituting said field-forming electrode is located between said spherical surfaces substantially normal to a line joining their centres, said sharp-edged conductor embracing both sides of said laminar member but being insulated therefrom by dielectric layers.

In a self-triggering spark-gap embodying the present invention said field-forming electrode may be electrically connected to said main electrode. The main electrode and said member may present cylindrical surfaces towards one another in spaced parallel relationship. The member may be electrically conducting to constitute said fieldforming electrode, and the sharp-edged conductor may be thin wire.

In a preferred form of self-triggered spark-gap said trigger electrode is a thin conducting sheet having a sharpedged hole therein, said field-forming electrode is a thin conducting sheet overlapping a peripheral region of said hole, and said main electrode faces said hole in capacitive relationship with said field-forming electrode. Preferably the hole and field-forming electrode are circular and the main electrode presents a substantially spherical surface to said field-forming electrode substantially concentrio with the hole.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein FIGURE 1 is a partially diagrammatic elevation of a spark-gap embodying the present invention.

FIGURE 2 is a cross-section on the line IIII of FIGURE 1.

FIGURE 3 is a diagram of a circuit using a pair of the present spark-gaps.

FIGURE 4 is a diagrammatic longitudinal section of a modified form of trigger assembly.

FIGURE 5 is a diagrammatic elevation of another type of externally triggered spark-gap embodying the invention.

FIGURES 6 and 7 are perspective views of trigger electrode assemblies used in the spark-gap of FIGURE 5.

FIGURE 8 is a modification of the spark-gap of FIG- URE 5.

FIGURE 9 is a perspective view of an alternative form of trigger electrode assembly.

FIGURE 10 is a perspective 'view of a self-triggered spark-gap incorporating the assembly of FIGURE 9.

FIGURE 11' is a circuit diagram of a delay circuit incorporating the spark-gap shown in FIGURE 10.

FIGURE'lZ is a diagrammatic sectional elevationof preferred form of self-triggered spark-gap.

' FIGURES l and 2 show a spark-gap suitable for use at a working voltage of 40 kv. The main electrodes are a pair of steel spheres 1 and 2 approximately 0.7 in. in diameter mounted on methylmethacrylate (Perspex) pillars 3 and 4 which stand on a base 5. Equispaced from pillars 3 and 4 and parallel thereto is a similar pillar 6 whose spacing from the spheres is adjustable by means of a screw 7 running in a slot 8 in the base.

I Pillar 6, which is 1 inch in diameter, forms part of the trigger assembly and supports the following layered structure. The outermost layer is a trigger electrode 9 formed as a copper band 0.003 in. thick and 0.15 inch wide fastened tightly round the convex surface of the pillar to prevent it lifting from the adjacent layer in use. Band 9 is insulated by a 0.003 inch thick sheet 10 of polyethylene terephthalate (Mylar) from a rectangular copper field-forming electrode 11, of similar thickness, which extends about halfway round pillar 6. Electrode 11 is insulated by a sheet 12, similar to sheet 10, from a further copper sheet 13 of similar dimensions which is in contact with the pillar itself. It will be seen that band 9 lies in a common diametral plane of the spheres, with its surface normal to that plane.

In use the field-forming electrode 11 is connected via a tab 14 to the mid-point of a potentiometer chain comprising resistors 15 connected between spheres 1 and 2. Sheet 13 is connected via tab 16 to sphere 2. A trigger pulse is applied via an 8:1 step-up transformer 17 between electrodes 9 and 11, the former of which is provided with a tab 18. A resistor 19 is connected between tabs 14 and 18.

In the static condition, prior to the application of a trigger pulse, sphere 1 is at 40 kv. (positive or negative) and sphere 2 is earthed. The trigger electrode 9 is therefore at 20 kv. relative to each sphere and the spacing is such that this is insuflicient to break down the gap between electrode 9 and either sphere.

On the application of a trigger pulse between electrodes 9 and 11, an intense electric field is created at the edges of band 9 where it lies over electrode 11, because the separation between them is so small. As a result of this field, corona is rapidly formed in copious quantities at these edges. The polarity of the trigger pulse is chosen to supplement the standing voltage between electrode 9 and sphere 2, and in the presence of the corona the gap between band 9 and sphere 2 becomes overvolted and breaks down. The function of sheet 13 is to rpovide a capacitance between electrode 11 and sphere 2, thereby stabilising the potential of electrode 11 during the pulse. Resistor 19 acts to damp out any overswing of the trigger pulse.

The breakdown of the gap between electrode 9 and sphere 2 causes the gap between electrode 9 and sphere 1 to become overvolted (no having 40 kv. across it) and hence to break down.

When the above-described spark-gap was fired by an 8 kv. trigger pulse (a 1 kv. input pulse to transformer 17) having a rise-time of 30-40 nanosecs, the breakdown was delayed by only about 70 nanosecs from the start of the trigger pulse. The jitter (i.e. the variation about this figure) was only 5 nanosecs. By using a 12 kv. trigger pulse, the jitter could be reduced to about 1 nanosec. The risetime of the current in a 50 ohm cable constituting an external load was 7-8 nanosec. The input capacity between electrodes 9 and 11 is only about 30 p.f.s., and hence the energy required in the trigger pulse is very small as compared vvith other gas switches (ignitrons, thyratrons etc.) of comparable performance.

Spark-gaps of similar configuration but increased dimensions have been used at 100 kv. In this case, however, it is impracticable to use sheet 13 to stabilise the potential of electrode 11 during the trigger pulse, or to use a separate capacitor for the purpose as in other forms of the invention to be described, because with such arrangements a damagingly high voltage can appear across the insulation between electrodes 9 and 11 when the spark-gap breaks down, owing to this capacitance remaining charged. It is possible, however, to use as this capacitance the stray capacitance which exists between electrode 11 and sphere 2 across the gap, which is automatically discharged by breakdown of the gap, and FIGURE 3 shows a circuit operating in this way.

This circuit comprises two storage capacitors 20 and 21 charged to +100 kv. and l kv. respectively via resistors 22 and 23, which are to be discharged into a load 24. The circuit is symmetrical about earth and employs two spark gaps 25 and 26 having spheres 1', 1", 2' and 2", and electrodes 9', 9", 11' and 11" respectively. Electrodes 11 and 11" are held at +50 kv. and 50 kv. respectively by potentiometer resistors 27 and 28.

The electrodes 9 and 9" are connected by capacitors 29 and 30 to opposite ends of the winding of a :1 stepup air-cored auto-transformer 31 having an earthed centretap. The central turns of this transformer are fed from a parallel-strip transmission-line pulse-generator 32 of the kind described in copending US. application Ser. No. 249,873 filed Jan. 7, 1963, now Patent No. 3,225,223, triggered by a swtich 33, which delivers a 15 kv. output pulse. Trigger pulses of 75 kv. amplitude and opposite polarities are therefore applied to electrodes 9 and 9".

Because the stray capacitance between electrodes 11' and 11 and the spheres is so small, the form of trigger assembly shown in FIGURE 1 is unsuitable, since only a relatively smallfraction of the applied pulse would appear across the relatively large capacitance between electrodes 9 and 11. To reduce the latter capacitance, the construction shown in FIGURE 4 is used, in which the portion of electrode 11 directly below electrode 9' lies in a recess, the V inch deep space between electrode 11 and the 0.005 inch Mylar sheet 10' being filled with polyethylene 34. The periphery of electrode 9', which is of 0.012 inch beryllium-copper about 0.8 inch Wide, overlaps the edges of the recess by only about 0.05 inch. This form of construction reduces the inter-electrode capacitance to a low value, while continuing to provide the intense field at the edges of electrode 9' to produce corona. (Alternatively sheet 11 may be divided circumferentially into two halves, the inner edge of each lying just under band 9'; however this construction is less desirable because of effects arising at the sharp inner edges.) The high-value resistors 35 and 36 maintain the potential of electrodes 11' and 11" at 50 kv. prior to triggering the gaps. Four-inch diameter spheres are used spaced about 1 inch from electrodes 9' and. 9". The delay between the production of the transformer pulses and the breakdown of the gap was about 200 nanosecs, with a jitter of about 10 nanosecs.

In FIGURE 5 the spheres 101 and 102 are located on opposite sides of the trigger assembly. The latter comprises a thin copper trigger electrode 109 which embraces a laminar field-forming electrode 111 but as before is separated from it by thin sheet Mylar. Typical forms of electrodes 109 and 111 are shown in FIGURES 6 and 7, in which the 0.002 inch Mylar sheet is omitted for clarity. The electrode 111 is maintained at a suitable potential between those of electrodes 101 and 102 by .means of resistors 115. As before, when a trigger pulse is applied between electrodes 109 and 111, e.g. via a transformer 117 as shown, a corona discharge occurs at the edges of electrode 109 as a result of the intense field produced thereat, causing the gap between it and sphere 101 to break down, which in turn breaks down the other half of the main gap. A separate capacitor 113 stabilises the potential of electrode 111 during the trigger pulse in the manner already described.

For a 10 kv. gap of this kind working at of its breakdown voltage (a safe working voltage) it is found that for minimum trigger voltage and fast working (approx. 50 nanosecs delay), the two halves of the gap should not be equally spaced nor have equal voltages on them. The ratio of the gap breakdown voltages and of the applied voltages is made 2:1, and care is taken that the trigger pulse is of the correct polarity to overvolt the gap having the lower breakdown voltage. Under these conditions the triggering pulse should be greater than of the total gap voltage to give fast, reproducible breakdown.

The reason for this unsymmetricity may be explained as follows.

Suppose the breakdown voltage on the gap between electrodes 102 and 101 to be unity and that the gap is to break down at 0.85 of the breakdown voltage as a result of corona initiation. Suppose also that the ratio of the voltages on the gap between electrodes 101 and 109 (termed gap A) and on the gap (termed gap B) between electrodes 102 and 109 to be fzl respectively.

The volts units on gap A are and the volts units on gap B are units 0.85 units The breakdown voltage of gap A is f/f-l-l units and of gapBis 1/1+f units When gap A breaks down 0.85 volt units is applied to gap B.

Gap B will break down if f 0.85 f 1 Total minim-um volt units required to cause fast breakdown is volt units required to be supplied by trigger pulse is Since f=0.35,

Minimum trigger volts=.078 volt units.

This is about of the total gap volt units (0.8) and, since there are always losses in the system, this figure is in practice raised to of the total gap voltage.

FIGURE 8 shows a somewhat similar gap for use at 100 kv. The spheres 101' and 102' are made larger than for the 10 kv. gap of FIGURE and the breakdown voltages of the two halves of the gap are in the ratio 3:4. The electrode 111 is attached to a toroidal guardring 116, whose stray capacity to the spheres stabilises the potential of electrode 111 during the trigger pulse as in the circuit of FIGURE 3. Electrodes 111 and 109' are separated by 0.020 inch Mylar and a 20 kv. trigger pulse is used for fast, reliable operation.

Although the descriptions of FIGURES 5 and 8 refer to the use of unequal voltages on the two halves of the gap, this is not, of itself, a novel arrangement, and it will be recalled from the descriptions of FIGURES 1-4 (in which equal voltages are applied to the two halves) that it is by no means an essential feature of the invention.

The cylindrical trigger assembly shown in FIGURE 9 is a modified form of those shown in FIGURES 1-4. The 6 Perspex cylinder 106 has a segment of its surface covered with a strip of copper sheet 211 and the whole is wrapped in 0.001 inch Mytlar sheet (not shown). On top of the Mylar is wound 0.005 inch dia. copper wire 209, securing to the Mylar, and in electrical contact with, a strip of copper sheet 114. The trigger voltage is applied between strips 211 and 114, giving rise to corona discharge from these portions of the wire 209 which lie above the strip 211.

A trigger assembly of the type shown in FIGURE 9 has been used in a 3 kv. gap of the kind shown in FIG- URES 5 and 8. Another application is the self-triggered gap shown in FIGURE 10. In this example the cylinder 106' is of metal wrapped with Mylar (not shown), thus dispensing with the need for strip 211 in FIGURE 9.

Spaced from cylinder 106' but mechanically and electrically connected to it by strip 122, is a metal cylinder 121. The gap is defined between wires 209 and cylinder 121. Where used in the delay circuit of FIGURE 11, the portions of the spark-gap of FIGURE 9 are shown diagrammatically but with the same reference numbers.

Referring to FIGURE 11, initially the capacitor 122 is charged via resistor 126 from. a 3 kv. source. The circuit is triggered by firing the fast gap 125, which is of the type shown in FIGURES 5 "and 8, whereupon capacitor 122 discharges through resistor 124 at a controllable rate. As the voltage on capacitor 122 falls, the voltage between wires 209' and cylinder 121 of the self-triggered gap increases until corona takes place at the surface of the wires and the gap breaks down, at say, 1.5 kv. The standard error in delay achieved is 3-4% for delays up to 10 ,usec and 6-7% up to 1000 ,usec.

Another form of self-triggered gap is shown in FIG- URE 12. It comprises a thin copper sheet 309 having a circular hole 350. A circular electrode 311, also made of thin copper sheet, is separated from sheet 309 by a thin sheet of Mylar 310, the whole lying on an insulating block 306. The diameter of electrode 311 slightly exceeds that of hole 350 so that there is a peripheral overlap of 0.05- 0.1 inch. Opposite the centre of the hole is mounted a sphere 321whose distance from the hole can be adjusted by a screw 322.

- In operation the potential of sheet 309 is held steady e.g. at earth, and an increasing potential is applied to sphere 321. As the potential of the sphere rises, the capacitance between it and sheet 311, which is electrically floating, raises the potential of the latter relative to sheet 309 causing corona to be produced at the sharp-edged circumference of the hole 350. Eventually the gap between sphere 321 and sheet 309 becomes overvolted in the presence of the corona and breakdown takes place between the sphere and the edge of the hole. For a gap intended to break down at 25 kv., sphere 322 was about 1 inch in diameter, sheets 309, 310 and 311 0.003 inch thick and hole 350 about ,4 inch in diameter. The spacing between the sphere and the sheets was about 0.4 inch. The stability of this gap is similar to that of FIGURE 10.

Instead of relying on the capacitative potential divider formed by the capacitances between sphere 321 and sheet 311 and between sheet 3 11 and sheet 309 to determine the potential of sheet 311, a resistive potential divider can be connected between the supply to sphere 321 and earth to do so. Such .an arrangement is particularly useful for gaps working at the lower voltages.

I claim:

1. A spark-gap including a trigger assembly arranged in spaced relationship with a main electrode in the gap, said assembly comprising a trigger electrode having a sharp edge exposed to the gap and a field-forming electrode separated from said exposed sharp edge by a thin layer of dielectric material to produce a corona-producing electric field at said sharp edge by application of a triggering voltage between said trigger electrode and said field-forming electrode, said dielectric material extending beyond said sharp edge to prevent the formation of a further gap extending from a conducting surface of said trigger electrode to a conducting surface of said fieldforming electrode and thereby prevent electrical breakdown occurring between said trigger and field-forming electrodes on application of said triggering voltage.

2. A spark-gap as claimed in claim 1 wherein said assembly comprises a member presenting a surface to said main electrode and being electrically conducting over at least a portion of said surface, said portion constituting said field-forming electrode, and a sharp-edged conductor overlying said portion but insulated therefrom by said dielectric layer to constitute said trigger electrode.

3. A spark-gap as claimed in claim 2 wherein said member is made of insulating material, and said portion is a thin conducting sheet applied to said member.

4. A spark-gap as claimed in claim 2 wherein said surface is' convex.

5. A spark-gap as claimed in claim 4' wherein the sharpedged conductor is thin wire.

6. A spark-gap as claimed in claim 4 wherein the sharpedged conductor is a thin conducting band.

7. A spark-gap as claimed in claim 4 comprising two main electrodes presenting substantially spherical surfaces to said trigger assembly, wherein the sharp-edged conductor is a thin conducting band located substantially in and normal to a common diametral plane of the spherical surfaces.

3. A spark-gap as claimed in claim 7 wherein the thin conducting sheet below a non-peripheral region of said .band is recessed into said member to reduce the capacitance between band and sheet.

9. A spark-gap as claimed in claim 7 wherein a further thin conducting sheetis insulatedly located between said thin conducting sheet and said member to form a capacitor therewith.

10. A spark-gap as claimed in claim 2 comprising a pair of main electrodes presenting substantially spherical surfaces to said trigger assembly, wherein a laminar conducting member constituting said field-forming electrode is located between said spherical surfaces substantially normal to a line joining their centres, said sharp-edged conductor embracing both sides of said laminar member but being insulated therefrom by dielectric layers.

11. A self-triggering spark-gap comprising a sparkgap as claimed in claim 2 wherein said field-forming electrode is electrically connected to said main electrode.

12. A spark-gap as claimed in claim 11 wherein said main electrode and said member present convex surf-aces towards one another in spaced parallel relationship.

13. A spark-gap as claimed in claim 12 wherein the member is electrically conducting to constitute said fieldforming electrode, and the sharp-edged conductor is thin wire.

14. A self-triggered spark-gap comprising a sparkgap as claimed in claim 1 wherein said trigger electrode is a thin conducting sheet having a sharpedged hole therein, staid field-f0nning electrode is a thin conducting sheet overlapping a peripheral region of said hole, and said main electrode faces said hole in capacitative relationship with said field-forming electrode.

15. A self-triggered spark-gap as claimed in claim 14 wherein the hole and field-forming electrode are circular and the main electrode presents a substantially spherical surface to said field-forming electrode substantially concentric with the centre of the hole.

References Cited UNITED STATES PATENTS 2,935,648 5/ 1960 Buntenbach 313306 X 2,997,623 8/1961 Westendorp 313-306 X 3,207,947 9/1965 Goncz 313306 X 3,260,883 7/1966 Martin 313306 X 3,267,320 8/ 1966 Melhart 313-306 X JOHN W. HUCKERT, Primary Examiner.

A. 1. JAMES, Assistant Examiner. 

1. A SPARK-GAP INCLUDING A TRIGGER ASSEMBLY ARRANGED IN SPACED RELATIONSHIP WITH A MAIN ELECTRODE IN THE GAP, SAID ASSEMBLY COMPRISING A TRIGGER ELECTRODE HAVING A SHARP EDGE EXPOSED TO THE GAP AND A FIELD-FORMING ELECTRODE SEPARATED FROM SAID EXPOSED SHARP EDGE BY A THIN LAYER OF DIELECTRIC MATERIAL TO PRODUCE A CORONA-PRODUCING ELECTRIC FIELD AT SAID SHARP EDGE BY APPLICATION OF A TRIGGERING VOLTAGE BETWEEN SAID TRIGGER ELECTRODE AND SAID FIELD-FORMING ELECTRODE, SAID DIELECTRIC MATERIAL EXTENDING BEYOND SAID SHARP EDGE TO PREVENT THE FORMATION OF A FURTHER GAP EXTENDING FROM A CONDUCTING SURFACE OF SAID 